1. Field of the Invention
The present invention relates generally to a photometric apparatus and photometric method and, more particularly, to a photometric apparatus and photometric method for obtaining photometric values by making use of an integrator capacitor that integrates a photocurrent produced from an optical sensor.
2. Description of the Related Art
A conventional photometric apparatus for obtaining photometric values by utilizing an integrator capacitor that integrates a photocurrent produced from a light-receiving device such as an optical sensor is disclosed, for example, in Japanese Patent Laid-Open No. 324328/1992. In the conventional technique disclosed in the specification of this laid-open publication, whenever charging voltage for an integrator capacitor charged with a photocurrent reaches a reference voltage, a pulse is produced. A photometric value is obtained either based on the number of pulses produced for a given time or from the time in which the charging voltage reaches the reference voltage.
However, where a photometric value is obtained based on the number of pulses produced in a given period with this conventional example, the same number of pulses are produced, whether the given time passes immediately after a certain pulse is generated or the given time passes immediately prior to generation of the next pulse. Therefore, the same photometric value is indicated, although the actual amounts of light incident on the optical sensor differ in practice. That is, the conventional example has the problem that the same photometric value is output if actual amounts of brightness (amounts of light) are different, thus providing poor accuracy. Where the time taken to reach the reference voltage is measured and the amount of incident light is calculated from the measuring time, if the amount of light incident on the optical sensor is small, a long time is necessary. Hence, the efficiency is low.
It is an object of the present invention to provide a photometric apparatus and photometric method capable of measuring a wide range of incident light amounts accurately and efficiently.
To achieve the above-described object, the present invention provides a photometric apparatus comprising: an optical sensor for generating a photocurrent according to the amount of incident light; an integrator capacitor for integrating the photocurrent from the optical sensor; a voltage detection means for detecting the voltage across the integrator capacitor; an integration time-measuring means for measuring an integration time for which the integrator capacitor integrates the photocurrent; resetting-and-controlling means for resetting the integrator capacitor whenever the voltage across the integrator capacitor exceeds a given voltage value; a totalizing means for finding a total voltage value integrated by the integrator capacitor during a given time; and a photometric means for finding a photometric value based on the total voltage value and on the integration time from the integration time-measuring means.
According to the configuration of the present invention described above, the photometric value is found based on the total integrated voltage value and integration time, the total integrated voltage value being owing to the photocurrent integrated by the integrator capacitor for the given time. Therefore, accurate photometry can be carried out. Furthermore, whenever the voltage across the integrator capacitor exceeds the given voltage, the integrator capacitor is reset. Also, the total voltage value integrated by the integrator capacitor for the given time is found. A voltage value exceeding the capacitance of the integrator capacitor is obtained as the total voltage value. Consequently, the capacitance of the integrator capacitor can be reduced. If an integrator capacitor with a small capacitance is used, high-resolution photometry can be performed.
Furthermore, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that the integration time measured by the integration time-measuring means during the given time is made equal to the given time subtracted by the time for which the resetting- and-controlling means are resetting the integrator capacitor during the given time.
In this configuration, the actual integration time within the given time can be found accurately. Therefore, accurate photometry can be carried out.
In addition, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that the totalizing means accumulates the difference between the voltage value of the integrator capacitor at the start of integration and the voltage value of the integrator capacitor immediately prior to resetting done by the resetting-and-controlling means after the start of the integration on each resetting during a given time to thereby obtain a sum value and that the difference between the voltage value of the integrator capacitor after a lapse of a given time and the voltage value of the integrator capacitor at the start of integration is added to the aforementioned sum value, thus deriving the total voltage value.
This configuration makes it possible to find the total voltage value accurately, the total voltage value being owing to integration by the integrator capacitor within the given time. Hence, accurate photometry can be carried out.
Further, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that the given time is 9.2 milliseconds or an integral multiple of it.
In this configuration, execution of accurate photometry is enabled if the amount of incident light varies (e.g., alternates) when the light is alternating light such as light from a fluorescent lamp.
Additionally, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that a voltage detection means has a detector for detecting the voltage across the aforementioned integrator capacitor at regular intervals of time. In this apparatus, the given voltage for the resetting-and-controlling means is set lower as the amount of incident light is increased.
In this configuration, if the amount of incident light is large and the voltage integrated by the integrator capacitor rises at a high rate, the voltage across the integrator capacitor is prevented from reaching the saturation range in the detection range of the detection portion or the saturation range of the integrator capacitor itself on detection of the detection portion. In consequence, accurate photometry can be performed.
Further, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that at least two integrator capacitors having different capacitances are included and that these capacitors are selectively used, only one at a time, according to the amount of incident light.
In this configuration, the capacitance of the integrator capacitor can be selected according to the amount of incident light. Therefore, high-resolution photometry can be carried out.
Further, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that the voltage detection means has an A/D converter for converting the voltage value across the integrator capacitor into a digital value at regular intervals of time except during resetting performed by the resetting-and-controlling means and that the integration time-measuring means measures a product as an integration time, the product being obtained by multiplying the number of A/D conversions made by the A/D converter within a given time by a certain period.
This configuration makes it possible to accurately find the actual integration time within the given time. Consequently, accurate photometry can be performed.
Further, the present invention provides a photometric apparatus that is based on the above-described configuration and further characterized in that at least two optical sensors are included and that these sensors are selectively used, only one at a time. This configuration can cope with multiple-split photometry.
Another aspect of the present invention provides a photometric method comprising the steps of: (a) producing a photocurrent according to the amount of incident light; (b) integrating the photocurrent; (c) detecting the voltage obtained by integrating the photocurrent; (d) resetting the integration whenever the integrated voltage exceeds a given voltage and repeating the steps (b), (c), and (d); (e) finding a total voltage value integrated during a given time; (f) measuring the time for which the photocurrent is integrated; and (g) finding a photometric value based on the integration time and on the total voltage value. This method involving these steps permits execution of accurate photometry.